The downscaling of element structures and interconnect structures is being pursued to achieve higher performance, higher capacity, and lower costs of semiconductor devices. For example, in a semiconductor memory device having a floating gate structure, downscaling of the arrangement pitches of the floating gates and the metal interconnects is progressing. Reducing the space between the gates, the space between the interconnects, or the element-separating region may cause problems due to the increase of the coupling capacitance, e.g., fluctuation of the threshold current of memory cells.
Therefore, an air gap structure has been proposed in which air (relative dielectric constant ∈=1) is provided in the space between the gates, in the space between the interconnects, or in the element-separating region because the relative dielectric constant of air is lower than that of a silicon oxide film (relative dielectric constant ∈=3.9) which is widely used as the insulating film material in the space between the gates, in the space between the interconnects, or in the element-separating region.